Embodiments of the subject invention relate to a method and apparatus for controlling a thermal environment of a payload, such as a thermal sensitive product. Embodiments are designed to move heat within an enclosed environment, such as a packaging system for temperature sensitive products, in order to more efficiently use the cold bank (or hot bank) and reduce hot or cold spots inside the packaging system.
The most common packaging systems for transporting temperature sensitive products use an insulated container, such as a Styrofoam container, and a cold bank, such as one or more frozen gel, or ice, packs, to provide thermal protection for a load, product, or payload. Typically, an environment of the load is maintained in a specific temperature range, such as 2-8° C., 20-25° C., or below −20° C., in order to provide thermal protection of the load. The frozen ice packs are typically placed on the top of the load, on the bottom of the load, or on the top and the bottom of the load. FIGS. 1A and 1B show two configurations that utilize the cold bank at the top of the load. In the packaging systems, cold air is moved throughout the container via natural convection to maintain an adequate temperature range, such as 2-8° C. Further, some packaging systems utilize channels on the inside walls, or spacers, to promote natural convection for uniform temperature distribution.
One of the major problems encountered with packaging systems similar to the packaging systems shown in FIGS. 1A-1B, using an insulated container and one or more cold banks, is poor temperature distribution, resulting in uneven temperatures throughout the internal environment of the insulated container. Since cold air is heavier than warm air, the cold air tends to settle at the bottom of the interior of the container, or package, which can expose the portions of the product positioned at the bottom of the interior of the container to freezing conditions. In addition, as warm air rises to the top, the portion of the product at the top of the interior of the package can be exposed to warm temperatures, which can result in the product not maintaining a proper temperature environment. By placing the cold bank at the top of the interior of the container, natural convection will tend to circulate the air within the container and mix the air naturally, allowing for a more uniform temperature distribution. However, a common industry practice is to fill the excess space inside the package with a filling material, such as bubble wrap or paper, in order to prevent shifting of the product and/or packaging components during handling. By filling in the excess space within the interior of the container with filling material, convective flow is diminished, which can result in temperature stratification within the interior of the container and, therefore, products being exposed to too warm and/or too cold temperatures.
Natural convection can be further limited if the packaging system is rotated, which can result in the cold bank not remaining in the desired location. As an example, rotating a container with a cold bank at the top of the interior of the container can result in the cold bank being positioned on the side or at the bottom of the interior of the container. Some packaging systems are used for shipping, often resulting in the container being flipped and/or rotated approximately 20 times during transport (Dea, 2004), resulting in a small probability that the package remains upright during the entire transit. Dea et al. (2006) reported that traditional packaging systems can suffer a reduction in the amount of time in the product experiences the appropriate temperature range by as much as 60% when the container is placed on the container's side or the container is placed upside down during transit.
A more uniform temperature distribution within the interior of the container promotes better thermal protection for products within the container by eliminating significant temperature gradients. A more uniform temperature distribution also allows for optimal effectiveness of cold banks within the container. By optimizing the internal temperature distribution, the cold bank can efficiently use its ability to absorb heat to maintain the product at the proper temperature. Inadequate temperature distribution reduces the effectiveness of the cold bank as the heat from the external environment transfers more to the cold source.
In order to eliminate or reduce temperature gradients, phase change materials have been added to the walls of the packaging system. In U.S. Pat. No. 7,328,583, issued Feb. 12, 2008, and in U.S. Pat. No. 7,849,708, issued Dec. 14, 2010, both disclosed a container with walls filled with phase change liquid (such as water) in order to provide a more uniform temperature inside the main container. Lining the walls with phase change material can reduce the temperature stratification when the package is flipped during transit. Phase change packaging systems can offer more thermal protection as they provide an additional insulative barrier between the external environment and the internal environment. Common phase change materials are water or vegetable oil based, which have very low thermal conductivities. However, phase change materials with shipping systems increases the cost of the insulated shipping container system, adds additional weight to the insulated shipping container that increases the shipping cost, requires more preparation time as the phase change materials need to be conditioned to the proper temperature before being placed inside of the package system, and often require the containers to be reused due to the higher cost.
Accordingly, there is a need for a method and apparatus for packaging temperature sensitive products in order to increase the amount of time the product experiences a desired temperature range and/or reduce the amount of time the product experiences temperatures outside of the desired temperature range.